The present invention concerns a process for the production of hollow bodies from thermoplastic material by blow molding and fluorination.
There are a number of situations involving containers made from a thermoplastic material, where there may be a wish or a need to influence the permeation properties of the wall of such a container and make the wall less permeable in relation to certain substances. Such a situation is for example that of a fuel tank which is installed in a motor vehicle. For that purpose, the container of thermoplastic material and in particular a container consisting of a polyolefin such as HDPE may be treated with a gas mixture containing fluorine in order to modify the permeation properties of the container wall.
A process for the production of hollow bodies from plastic material is disclosed in German laid-open application (DE-OS) No 34 35 392 and U.S. Pat. No. 4,617,077, in which the hollow body is produced by means of extrusion blow molding and fluorination, the gas mixture used for the fluorination treatment being oxygen-free. In another process, as disclosed in German laid-open application (DE-OS) No 36 37 459 and German patent specification No 3 840 269, the permeation properties of containers comprising polymers of ethylene or propylene can be improved by a treatment involving the use of a gas mixture which, besides fluorine, also contains oxygen.
While all the above-discussed processes for influencing the permeation properties of walls of hollow bodies made from plastic material using fluorine, for example in the case of fuel tanks, give rise to a noticeable improvement in such properties in terms of reducing the degree of permeability of the container wall in relation to fuels or components thereof, it is difficult, in consideration of the fact that the relevant applicable requirements are steadily becoming more and more strict, to achieve the values which in future times will still be admissible, in regard to the amounts of fuel or fuel components which escape from such a container or tank by diffusion, per unit of time. In that respect the composition of the fuel also plays a considerable part as for example the permeation properties of fluorinated polymers vary in dependence on the alcohol content and in particular the methanol content of the fuel contained in a container made from such a material. Thus, when the fuel contains certain proportions of methanol, the permeability of such fuel tank walls markedly increases, with the result that it is frequently no longer possible to comply with the limit values prescribed in many if not most countries. The endeavour is to reduce the degree of permeation to very low values, for example less than 0.3 g/24 hours per tank, even in relation to alcohol-bearing fuels. That is something that can scarcely be achieved with the fluorination procedures which are usually employed nowadays. A further difficulty in this respect is that it is necessary to reckon on the composition of the fuel fluctuating, for example in such a fashion that, during the service life of a normal vehicle and the tank fitted therein, the tank my be filled with fuels of differing compositions, including fuels with variations in regard to the proportion of alcohol therein.